[katehovey]

I have a problem to solve on heat transfer over double glazing compared to single. I worked out the heat flow (Q) over the single glazing to be 12000W and over the double glazing to be 110.43W - which seems to be considerably high! And I'm stuck as to what I should do now.

The question is:

A window, 3m wide by 1.5m high, originally contained a single pane of glass 6mm thick. It is proposed to install double glazing consisting of two sheets of glass (each 6mm thick) separated by a stagnant air gap 20mm thick at a cost of £600. If heating costs 4.5p per kWh, estimate the time taken to recover the cost of the double glazing when the temperature difference between inside and outside surfaces of the glass window is 20^oC.

Thermal conductivity of glass = 0.8Wm^-1K^-1
Thermal conductivity of air = 0.025Wm^-1K^-1

Thanks.

[Donaldson Tan]

Your values are right.

You are loosing heat at 12000W with single glazing and 110.43W with double glazing, so

Heating Saved in an hour = (12000 - 110.43) X 1h = 11890Wh = 11.89kWh
Savings Generated in an hour = 11.89kWh X 4p/kWh = 47.56p/kWh = £0.4756/kWh
Time Taken to Recover Cost = 600/0.4756 = 1262h = 52.6 days

[katehovey]

Thank you very much.

Kate